Distant Quasar Confirms Universe Was Warmer 7 Billion Years Ago

An international team of astronomers led by Dr Sebastien Muller of the Chalmers University of Technology has made the most precise measurement ever of how the Universe has cooled down during its 13.77-billion-year history.

Thanks to cold gas clouds in a distant galaxy and radio waves from a quasar known as PKS 1830-211 behind it – the red point in the middle of the picture, astronomers have measured the temperature of the Universe 7.2 billion years ago (NASA / ESA / E. Falco / F. Courbin)

The astronomers using CSIRO’s Australia Telescope Compact Array – an array of six 22-m radio telescopes in eastern Australia – measured the temperature of the cosmic microwave background, the very weak remnant of the heat of the Big Bang that pervades the entire Universe.

They studied gas in an unnamed galaxy 7.2 billion light-years away.

“When we look at this galaxy with our telescopes, we see it as it was when the Universe was younger – and warmer – than it is now,” Dr Muller said. “The only thing keeping this gas warm is the cosmic background radiation – the glow left over from the Big Bang.”

By chance, there is another powerful galaxy, a quasar called PKS 1830-211, lying behind the unnamed galaxy.

Radio waves from PKS 1830-211 come through the gas of the foreground galaxy. As they do so, the gas molecules absorb some of the energy of the radio waves. This leaves a distinctive ‘fingerprint’ on the radio waves.

From this ‘fingerprint’ the astronomers calculated the gas’s temperature. They found it to be 5.08 Kelvin (minus 450.5 F): extremely cold, but still warmer than today’s Universe, which is at 2.73 Kelvin (minus 454.8 F).

“The temperature of the cosmic background radiation in the past has been measured before, at even larger distances. But this is the most precise measurement yet of the ambient temperature when the Universe was younger than it is now,” said study co-author Dr Alexandre Beelen of the University of Paris’ Institute for Space Astrophysics, France.

Radio waves from a bright quasar pass through a galaxy where they are absorbed by molecules in a cold cloud of gas. Astronomers used radio telescopes to measure the temperature of the Universe 7.2 billion years ago by measuring the signatures of the molecules in the radio waves (Onsala Space Observatory / R. Cumming / S. Muller)

According to the Big Bang theory, the temperature of the cosmic background radiation drops smoothly as the Universe expands. “That’s just what we see in our measurements. The Universe of a few billion years ago was a few degrees warmer than it is now, exactly as the Big Bang theory predicts,” Dr Muller concluded.